In the field of a construction machine such as a hydraulic excavator, there exists a closed circuit hydraulic drive system in which, in order to achieve energy saving for the hydraulic system, a both-way tilting hydraulic pump (hereinafter also referred to as the closed circuit pump) and a hydraulic actuator are connected to each other in a closed-circuit-like fashion and in which the drive speed of the hydraulic actuator is controlled through delivery flow rate control of the both-way tilting hydraulic pump to return the return fluid from the hydraulic actuator to the closed circuit pump (see, for example, Patent Document 1).
There is a failure diagnosis device detecting sticking failure of a hydraulic proportional valve (hereinafter also referred to as the proportional valve) used in hydraulic devices in general (see, for example, Patent Document 2). This failure diagnosis device for the proportional valve is a failure diagnosis device for the proportional valve of a vehicle hydraulic device having a pump, accumulator, and proportional valve, and is equipped with proportional valve drive direction means for outputting a proportional valve direct current for driving the proportional valve, pressure measurement means for measuring the pump pressure of the pump, and proportional valve sticking determination means for making sticking determination of the proportional valve based on the proportional valve direct current and the pump pressure. The proportional valve sticking determination means has first pressure storage means storing the pump pressure when the pump stops, second pressure storage means storing the pump pressure when a predetermined period of time has elapsed since the stopping of the pump, and first pressure difference calculation means calculating the difference between the pump pressure stored in the first pressure storage means and the pump pressure stored in the second pressure storage means; and in the case where when the pump is stopped and “close” direction is given to the proportional valve after increasing the pressure through the operation of the pump for a predetermined period of time, the difference in the pump pressure calculated by the first pressure difference calculation means is equal to or more than a predetermined value, it is determined that the proportional valve suffers from stuck-open failure.